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Vitamin C
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin. Unlike most mammals, humans do not have the ability to make their own vitamin C. Therefore, we must obtain vitamin C through our diet.
Function
Vitamin C is required for the synthesis of collagen, an important structural component of blood vessels, tendons, ligaments, and bone. Vitamin C also plays an important role in the synthesis of the neurotransmitter, norepinephrine. Neurotransmitters are critical to brain function and are known to affect mood. In addition, vitamin C is required for the synthesis of carnitine, a small molecule that is essential for the transport of fat to cellular organelles called mitochondria, for conversion to energy (1). Recent research also suggests that vitamin C is involved in the metabolism of cholesterol to bile acids, which may have implications for blood cholesterol levels and the incidence of gallstones (2).
Vitamin C is also a highly effective antioxidant. Even in small amounts vitamin C can protect indispensable molecules in the body, such as proteins, lipids (fats), carbohydrates, and nucleic acids (DNA and RNA) from damage by free radicals and reactive oxygen species that can be generated during normal metabolism as well as through exposure to toxins and pollutants (e.g. smoking). Vitamin C may also be able to regenerate other antioxidants such as vitamin E (1).
Deficiency
Scurvy
Severe vitamin C deficiency has been known for many centuries as the potentially fatal disease, scurvy. By the late 1700's the British navy was aware that scurvy could be cured by eating oranges or lemons, even though vitamin C would not be isolated until the early 1930's. Symptoms of scurvy include bleeding and bruising easily, hair and tooth loss, joint pain and swelling. Such symptoms appear to be related to the weakening of blood vessels, connective tissue, and bone, which contain collagen. Early symptoms of scurvy such as fatigue may result from diminished levels of carnitine, needed to derive energy from fat, or decreased synthesis of the neurotransmitter norepinephrine (see Function). Scurvy is rare in developed countries because it can be prevented by as little as 10 mg of vitamin C daily (2). However, recent cases have occurred in children and the elderly on very restricted diets .
The Recommended Dietary Allowance (RDA)
In the U.S., the recommended dietary allowance (RDA) for vitamin C was recently revised upward from 60 mg daily for men and women. The recommended intake for smokers is 35 mg/day higher than for nonsmokers, because smokers are under increased oxidative stress from the toxins in cigarette smoke and generally have lower blood levels of vitamin C.
Recommended Dietary Allowance (RDA) for Vitamin C
Life Stage Age Males (mg/day) Females (mg/day)
Infants 0-6 months 40 (AI) 40 (AI)
Infants 7-12 months 50 (AI) 50 (AI)
Children 1-3 years 15 15
Children 4-8 years 25 25
Children 9-13 years 45 45
Adolescents 14-18 years 75 65
Adults 19 years and older 90 75
Smokers 19 years and older 125 110
Pregnancy 18 years and younger - 80
Pregnancy 19-years and older - 85
Breastfeeding 18 years and younger - 115
Breastfeeding 19 years and older - 120
Coronary Heart Disease
Until recently, the results of most prospective studies indicated that low or deficient intakes of vitamin C were associated with an increased risk of cardiovascular diseases and that modest dietary intakes of about 100 mg/day were sufficient for maximum reduction of cardiovascular disease risk among nonsmoking men and women.study found that the risk of death from cardiovascular diseases was 42% lower in men and 25% lower in women who consumed more than 50 mg/day of dietary vitamin C and who regularly took vitamin C supplements, corresponding to a total vitamin C intake of about 300 mg/day. Results from the Nurses’ Health Study, based on the follow-up of more than 85,000 women over 16 years, also suggest that higher vitamin C intakes may be cardioprotective. In this study, vitamin C intakes of more than 359 mg/day from diet plus supplements or supplement use itself were associated with a 27-28% reduction in CHD risk. Data from the National Institutes of Health (NIH) indicate that plasma and circulating cells in healthy, young subjects became fully saturated with vitamin C at a dose of about 400 mg/day.The results of the pooled analysis of prospective cohort studies suggest that maximum reduction of CHD risk may require vitamin C intakes high enough to saturate plasma and circulating cells, and thus the vitamin C body pool.
Stroke
Additionally, the risk of stroke in those who consumed vegetables 6-7 days of the week was 54% lower than in those who consumed vegetables 0-2 days of the week. In this population, serum levels of vitamin C were highly correlated with fruit and vegetable intakes. Therefore, as in many studies of vitamin C intake and cardiovascular disease risk, it is difficult to separate the effects of vitamin C on stroke risk from the effects of other components of fruits and vegetables, emphasizing the benefits of a diet rich in fruits and vegetables.
Cancer
A large number of studies have shown that increased consumption of fresh fruits and vegetables is associated with a reduced risk for most types of cancer. A number of case-control studies have investigated the role of vitamin C in cancer prevention. Most have shown that higher intakes of vitamin C are associated with decreased incidence of cancers of the mouth, throat and vocal chords, esophagus, stomach, colon-rectum, and lung. In general, prospective studies in which the lowest intake group consumed more than 86 mg of vitamin C daily have not found differences in cancer risk, while studies finding significant cancer risk reductions found them in people consuming at least 80 to 110 mg of vitamin C daily (1).
Most large prospective studies found no association between breast cancer and vitamin C intake, two recent studies found dietary vitamin C intake to be inversely associated with breast cancer risk in certain subgroups. In the Nurses' Health Study, premenopausal women with a family history of breast cancer who consumed an average of 205 mg/day of vitamin C from foods had a 63% lower risk of breast cancer than those who consumed an average of 70 mg/day. A number of observational studies have found increased dietary vitamin C intake to be associated with decreased risk of stomach cancer, Infection with the bacteria, helicobacter pylori (H. pylori) is known to increase the risk of stomach cancer and also appears to lower the vitamin C content of stomach secretions. More recent research suggests that vitamin C supplementation may be a useful addition to standard H. pylori eradication therapy in reducing the risk of gastric cancer.
Lead toxicity
Abnormal growth and development has been observed in infants of women exposed to lead during pregnancy, while children who are chronically exposed to lead are more likely to develop learning disabilities, behavioral problems, and to have low IQs. In adults, lead toxicity may result in kidney damage and high blood pressure. A lower dose of 200 mg/day did not significantly affect blood lead levels, despite the finding that serum vitamin C levels were not different than those of the group that took 1,000 mg/day. The mechanism for the relationship between vitamin C intake and blood lead levels is not known, although it has been postulated that vitamin C may inhibit intestinal absorption or enhance urinary excretion of lead.
Sources
Food Sources
As shown in the table below different fruits and vegetables vary in their vitamin C content, but five servings (2½ cups) of fruits and vegetables should average out to at least 200 mg of vitamin C. If you wish to check foods you eat frequently for their nutrient content, search the USDA food composition database.
Food Serving Vitamin C (mg)
Orange juice ¾ cup (6 ounces) 75
Grapefruit juice ¾ cup (6 ounces) 60
Orange 1 medium 70
Grapefruit ½ medium 44
Strawberries 1 cup, whole 82
Tomato 1 medium 23
Sweet red pepper ½ cup, raw chopped 141
Broccoli ½ cup, cooked 58
Potato 1 medium, baked 26
Supplements
Vitamin C (L-ascorbic acid) is available in many forms, but there is little scientific evidence that any one form is better absorbed or more effective than another.
Natural vs. synthetic vitamin C
Natural and synthetic L-ascorbic acid are chemically identical and there are no known differences in their biological activities or bioavailability (41).
Mineral ascorbates
Mineral salts of ascorbic acid are buffered and therefore, less acidic than ascorbic acid. Some people find them less irritating to the gastrointestinal tract than ascorbic acid. Sodium ascorbate and calcium ascorbate are the most common forms, although a number of other mineral ascorbates are available. Sodium ascorbate generally provides 131 mg of sodium per 1,000 mg of ascorbic acid, and pure calcium ascorbate provides 114 mg of calcium per 1,000 mg of ascorbic acid.
Ascorbyl palmitate
Ascorbyl palmitate is actually a vitamin C ester (vitamin C that has been esterified to a fatty acid). In this case, vitamin C is esterified to the saturated fatty acid, palmitic acid, resulting in a fat-soluble form of vitamin C. Ascorbyl palmitate has been added to a number of skin creams due to interest in its antioxidant properties as well as the important role of vitamin C in collagen synthesis (43). Although ascorbyl palmitate is also available as an oral supplement, it is likely that most of it is hydrolyzed (broken apart) to ascorbic acid and palmitic acid in the digestive tract before it is absorbed (44). Ascorbyl palmitate is also marketed as, "vitamin C ester," which should not be confused with Ester-C® (see above).
Safety Toxicity
A number of possible problems with very large doses of vitamin C have been suggested, mainly based on in vitro experiments or isolated case reports, including: genetic mutations, birth defects, cancer, atherosclerosis, kidney stones, "rebound scurvy", increased oxidative stress, excess iron absorption, vitamin B-12 deficiency, and erosion of dental enamel. However, none of these adverse health effects have been confirmed, and there is no reliable scientific evidence that large amounts of vitamin C (up to 10 grams/day in adults) are toxic or detrimental to health. With the latest RDA published in 2000, a tolerable upper intake level (UL) for vitamin C was set for the first time. Such symptoms are not generally serious, especially if they resolve with temporary discontinuation or reduction of high-dose vitamin C supplementation.
Tolerable Upper Intake Level (UL) for Vitamin C
Age Group UL (mg/day)
Infants 0-12 months Not possible to establish*
Children 1-3 years 400
Children 4-8 years 650
Children 9-13 years 1,200
Adolescents 14-18 years 1,800
Adults 19 years and older 2,000
*Source of intake should be from foods or formula only.
Linus Pauling Institute Recommendations
The Linus Pauling Institute recommends a vitamin C intake of at least 400 mg daily—the amount that has been found to fully saturate plasma and circulating cells with vitamin C in young, healthy nonsmokers (13). Consuming at least five servings (2½ cups) of fruits and vegetables daily may provide about 200 mg of vitamin C. Most multivitamin supplements provide 60 mg of vitamin C.
Older adults (65 years and older)
Although it is not yet known with certainty whether older adults have higher requirements for vitamin C than younger people, some older populations have been found to have vitamin C intakes considerably below the RDA of 75 and 90 mg/day for women and men, respectively. A vitamin C intake of at least 400 mg daily may be particularly important for older adults who are at higher risk for chronic diseases.